This application outlines a 5 year plan to train the candidate for an academic career investigating the pathogenesis of hepatitis C virus (HCV) infection and mechanism of drug resistance. Dr. Wang has completed residency training in Internal Medicine at Yale University and a Clinical Fellowship in Infectious Diseases at University of Pennsylvania. He has a strong background in mechanistic enzymology, having received a Ph.D. in biochemistry under Dr. Charles Grubmeyer at Temple University, and is now working in the laboratory of his sponsor, Dr. Frederic Bushman, studying the function of lentiviral vectors in gene therapy patients using DMA bar coding and pyrosequencing. Dr. Wang has established a comprehensive research and training program so that he can gain the expertise to independently attack questions of HCV pathogenesis and resistance. Dr. Bushman, a world authority in the field of virology, and Dr. Kyong-Mi Chang, an expert in HCV pathogenesis, will jointly mentor the candidate's scientific development. An advisory board of prominent scientists and clinicians will mentor his career development. HCV is a leading cause of chronic liver disease in the U.S. The only FDA approved treatment using interferon is often ineffective, yet the basic mechanisms of resistance are largely unknown. Several compounds targeting viral-encoded enzymes are currently in clinical development. However, their use will almost certainly be complicated by the highly diverse viral populations in vivo, which may result in complex drug resistance pathways. Recently, the application of genome sequencing techniques to such problems in viral resistance has allowed mass identification of viral variants. In published work, we have utilized DNA bar coding and pyrosequencing to identify rare drug resistant mutations in HIV and to monitor locations of retroviral integration sites in vivo. We will combine these powerful tools with bioinformatics and a variety of molecular techniques to advance understanding of HCV evolution and resistance in vivo. We propose the following Aims: 1) establish methods for efficient amplification and high-throughput pyrosequencing of HCV; 2) investigate the selective pressures on viral populations resulting from interferon therapy; 3) define the population structure of HCV. The proposed in-depth laboratory and didactic training will guide the candidate in becoming an independent investigator in viral pathogenesis and resistance.